Conductivity, mobility and carrier density can be important parameters for characterization of electronic materials. Yet accurate measurements of materials can be difficult to achieve with standard methods. Firstly, time varying carrier density or mobility cannot be accurately characterized because multiple sequential measurements with different contact configurations are typically required. Time varying carrier density is known to occur in amorphous oxide semiconductors which are driving today's flat-panel industry, and proper characterization of such carrier density transients could help to identify and eliminate their detrimental effects. Secondly, low mobility materials can require unreasonably large magnetic fields to characterize using traditional Hall effect methods. Thirdly, low mobility materials tend to mix the Hall effect signal with a large, drifting background offset, making accurate measurements difficult. Low-mobility materials include conducting oxides for display front-plane interconnects and organic conductors for flexible electronics.
A traditional method of Hall measurement is to measure a pair of Hall resistances RH+ in a positive magnetic field and RH− in a negative magnetic field sequentially and calculate RH+−RH− to obtain the Hall resistance. This requires switching magnetic field polarity. Alternately, one can switch measurement contacts either manually or electrically. There is a prior non-switching van der Pauw technique that measures RH+ and RH− simultaneously, but each component still has a large offset, which introduces large measurement error for low mobility materials. An AC field method reduces noise from the large offset by modulating the magnetic field with a fixed frequency and measuring only at the desired frequency. However, the AC magnetic field can require a complex control unit including a mechanical motor to physically rotate the magnet or a precisely controlled electromagnet, taking a long time to conduct one measurement, and has limited maximum field strength. The AC field method is also incompatible with the non-switching van der Pauw technique, so in principle two sequential AC magnetic field sample configurations would need to be measured.